Image articles comprising security images are well known. Security images generally comprise an image which is invisible or undetectable under ambient conditions, and which can be rendered visible or detectable by application of a suitable stimulus; or alternatively, the image may change from one colour under ambient conditions to another colour upon application of a stimulus.
Security images may be manufactured by coating security inks or compounds onto a substrate. Examples of known compounds which when coated onto a substrate provide a security image include photochromic compounds which generally change from colourless to coloured upon the application of ultraviolet light, and thermochromic compounds which generally change from colourless to coloured upon the application of heat.
Image articles which include security images are useful in many areas of industry such as in packaging, identification cards, and labels, for example. It is useful to provide packaging which includes a security image invisible to a user under ambient conditions, but which can be rendered visible upon application of a stimulus; for example, if a customs and excise official wishes to check that imported goods are genuine goods or whether they are counterfeit goods. If the packaging includes the security image, rendered visible or detectable by suitable stimulus, the customs and excise official can determine that the packaging, and hence the goods, are not counterfeit. Likewise, it is advantageous to provide an identification card in which an image is invisible or a defined colour under ambient conditions, but which can be rendered visible or detectable, or change colour upon application of a stimulus in order to prove the identity of a user of the identity card, and in order to determine that the identity card is genuine and not a counterfeit identity card. There are many known examples of such security images, for example, in the applicant's co-pending applications PCT/GB2005/001763 and PCT/GB2005/001766.
In the manufacture of bank notes, it is desirable to include as many security features as possible, which may include multiple security images using a variety of compounds capable of changing colour upon application of a stimulus (including movement of the bank note to change viewing angle), or turning coloured from colourless, or vice versa.
In many countries, officials and state authorities use apparatus, such as third party verifiers, which detect the percentage light absorbance and/or reflection at a wavelength of approximately 800-900 nm (in the infrared region), to detect whether specific security images comprising compounds which absorb infrared radiation between 800-900 nm are present; and hence help to determine whether or not a bank note is genuine or counterfeit.
It is desirable to provide bank notes which contain security images comprising compounds capable of exhibiting 50% or less light reflectance at approximately 800-900 nm. Many bank notes include Carbon Black as a pigment which possesses the characteristic of less than 50% light reflectance at 800-900 nm. Unfortunately, in order to provide a suitably strong image, with the required light reflectance characteristics at 800-900 nm, Carbon Black is generally needed in a concentration which produces a dull grey image in the positions where the Carbon Black is located, when coated at concentrations generally used (for example, 3% w/w of the total weight of the ink dispersion laid down on the substrate paper for bank notes). Bank note counterfeiters recognise from the dull grey image that Carbon Black is present in bank notes, and commonly now use Carbon Black in order to avoid their counterfeit bank notes being detected as counterfeit when utilising third party verifiers to verify the light reflectance at 800-900 nm.
It would therefore be advantageous to provide a security image on a bank note or any other image article requiring a security image, in which a security image includes one or more compounds having a 50% or less light reflectance at 800-900 nm at a given concentration within an image (and preferably around 850 nm), and in which the compound utilised does not create a strongly coloured image. It would be particularly advantageous to provide such a compound for inclusion in a security image in which the compound produces a substantially colourless security image, but which has 30% or less light reflectance at 800-900 nm. Most preferably 10% light reflectance in the 800-900 region is desired.
It is therefore an aim of the preferred embodiments of the present invention to overcome or mitigate at least one problem with the prior art, whether expressly disclosed herein or not.